Shaft seals

ABSTRACT

A rotary mechanical seal assembly for a shaft comprises a freely floating ring of flexible material which has inner and outer axially directed lips to bear against the surfaces of the shaft and housing, at least one of these lips being normally out of contact with its associated surface so that no wear takes place. The seal is brought into action by the application of pressure either applied from an outside source or created by leakage past an adjacent upstream face-type seal for which the seal of the invention forms an emergency back-up. There may be a detector to sense the leakage. There could be two such lip seals facing one another.

This invention relates to shaft seals. Where a shaft is sealed by aknown kind of rotary mechanical face seal it may, in certain situations,be desirable to back it up with some form of emergency device which willat least provide a temporary seal, or a seal able to hold the pressureindefinitely under non-running conditions, in the event of failure ofthe main seal. Yet if this emergency seal is permanently in action itwill itself be subject to wear and may indeed fail before the main seal,being consequently useless at the very moment when it is needed.

It is therefore desirable that the emergency device should only comeinto action when it is needed, i.e. on failure of the main seal. The aimof the present invention is to provide a device which achieves this.

According to the invention a rotary mechanical seal assembly suitablefor use as an emergency seal and designed for a shaft mounted rotatablyin a housing comprises a ring of flexible material which is free tofloat axially on the shaft, or on an external cylindrical surfaceassociated with the shaft, and has inner and outer annular lips bothfacing in the same axial direction and capable of engaging and sealingrespectively against the shaft or said surface on the one hand andagainst an internal cylindrical surface associated with the surroundingstationary housing on the other hand, the dimensions being such that inits free condition the ring is normally out of contact with the shaft orsaid external surface on the one hand and/or with the internal surfacebut comes into contact with both, to form a seal, when fluid pressureacts against that axial face of the ring towards which the lips aredirected.

The ring may be of hammer-head cross-section, that is to say, with acentral portion which is relatively rigid, with inner and outer limbswhich form the lips and are more flexible.

In one example the dimensions are such that the inner lip of the ring isan interference fit on the shaft, so that the ring rotates with theshaft, whilst its outer diameter has a positive clearance from theadjacent internal cylindrical surface of the housing under normalconditions. However it is preferred to make the ring an interference fitin the housing and provide a positive clearance between its innerdiameter and the shaft; the first mentioned version does have the meritthat it intercepts any slight leakage of oil along the shaft and flingsit clear by centrifugal force.

There is preferably also a radial surface (preferably on the housing butit could be a shoulder on the shaft) against which the downstream faceof the ring comes into engagement when the ring moves axially underfluid pressure. This provides an extra seal and also the engagement ofthe ring against this surface helps the ring to expand radially to formthe seal.

The invention will now be further described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a section through a rotary mechanical face seal assemblyfitted with the emergency seal assembly according to the invention;

FIG. 2 is a section through the emergency seal element alone, in itsfree condition, to a larger scale;

FIG. 3 is a section to a still larger scale than FIG. 2, showing theemergency seal element in its normal fitted condition and engaged by itssupport ring;

FIG. 4 is a view similar to FIG. 1 showing a modified seal faceassembly, and

FIG. 5 shows yet another seal face assembly.

Referring first to FIG. 1 this shows what is basically a known kind ofrotary mechanical face seal assembly in which a shaft 1, to be sealedinto a housing 2, carries a rotary seal face member 3 which is urgedaxially by helical coil compression springs, of which one is visible at4, into rubbing contact with a stationary seat 5 which is sealed intothe housing 2. The seat 5 is received in a clamping ring 6 secured tothe face of the housing. A portion of the fluid product against whichthe seal is acting is fed under pressure through a restriction 7 andthence through connection 7 into the housing 2, so as to cause theproduct to circulate around the seal.

The emergency seal according to the invention comprises primarily anannular seal element 8 made of a suitable synthetic rubber or, betterstill, of PTFE. The profile of the element as made is shown in FIG. 2and it will be seen that it comprises a radially outwardly projectinglip 9 and a lip 10 that projects inwards at an inclination to the axisof the element, together with a central axially extending cylindricalportion 11 which is thicker than the two lips and is therefore lessflexible. The result is a profile of hammer-head form. This portion 11serves to stiffen the element as a whole, enabling it to withstand highpressures, and it also prevents the lips 9 and 10 being damaged bycontact with the end face of the space in which the element is received,described below.

The seal element 8 is received in an annular space 12 defined between acounterbore in the outer face of the clamping ring and a further ring 13secured to it. Also within the space 12 is a rigid metal support ring 14with an axially projecting lip 15 that engages the inside diameter ofthe cylindrical portion 11 of the seal element; this support ring servesto locate the element 8, radially during assembly, and also to stiffenit further.

In the example shown the shaft 1 is protected, in the region of theseals, by a sleeve 16 that is held in place by a ring 17 clamped to theshaft by grub screws 18. A split journal bearing 19 within the furtherring 13 runs on the sleeve 16, for the purpose of complying withregulations that require restriction on the rate of leakage in the eventof total failure of the seals.

The inner diameter of the emergency seal element 8 is such that undernormal operating conditions the lip 10, which extends inwards in aninclined direction towards the main seal, is just out of contact withthe external surface of the sleeve 16. However the outside diameter ofthe element 8, in its free condition is substantially greater than thediameter of the space 12 and so, as will be seen in FIG. 3, the lip 9 isdeflected to a substantial degree and bears against the outside surfaceof the counterbore that defines the space 12 to form a sealing lipdirected axially inwards, i.e. towards the main seal 3,5.

Under normal operating conditions, therefore, with the main sealperforming its correct function of holding back the product in thehousing 2, the emergency seal element 8 is stationary and has nofunction. Moreover it is not subject to any wear as the lip 10 is clearof the sleeve 16. Any slight leakage of fluid past the main seal is ableto pass also the lip 10 and escape through a drain, not shown, from thelowest point of the left-hand end (as viewed in FIG. 1) of the space 12.

In the event of partial or complete failure of the main seal, however,the flow of fluid along the outside of the sleeve 16 will be sufficientto act on the lip 10 to deflect it into contact with the sleeve 16,forming an effective lip seal. The greater the pressure, the greater isthe force with which the lip is held in sealing contact with the sleeve.In the example shown the sleeve 16 is provided with a deposit 20 of hardmaterial to prevent local wear.

At the same time the element 8, which is normally floating within thespace 12, is urged hard up against the left-hand end of that space andthe outer lip 9 is urged hard against the outside surface of the space.The radial surface at the left-hand end of the space, against which theelement 8 is pressed, itself forms a further seal in conjunction withthe element 8.

The fluid that builds up in the space between the main seal and theemergency seal is allowed to escape to a limited extent via a connection21 leading through a restriction 22 to drain (not shown) and at the sametime the pressure that builds up is detected by a fluid pressure switch23 which closes electrical contacts to signal an alarm or to takewhatever other steps are required to cope with failure of the seal. Inthe meantime the shaft 1 is able to continue running for a substantialtime without difficulty, thanks to the fact that the emergency sealelement 8 is in a substantially unworn condition up to the moment offailure of the main seal. Moreover the emergency seal is able to holdback the pressure for an unlimited period when the shaft is not running.

The sleeve 16 may be omitted altogether and the seal element 8 couldthen, in an emergency, bear directly on the shaft. In a furthermodification the seal element 8 could have an inner lip normally incontact with the shaft (or sleeve) whilst its outer lip is normally justclear of the housing. In that case the element 8 would rotate with theshaft under normal running conditions but would be brought to a halt bythe pressure build-up on failure of the main seal. This version has theadvantage that slight leakage along the shaft from the main seal isflung off centrifugally by the rotating seal element 8 and thereforedoes not pass further along the shaft.

Instead of, or in addition to, the pressure switch 23 there could bemeans for detecting and signalling fluid flow rather than fluidpressure, but for the same purpose of drawing attention to failure ofthe main seal.

Finally, it would be possible to design the assembly so that both theinner and outer lips of the element 8 are out of contact with theirrespective engaging surfaces under normal running conditions but bothcome into engagement with the surfaces when pressure develops upstreamof them.

FIG. 4 shows diagrammatically a layout similar to that shown in FIG. 1but with two of the members 8 arranged facing opposite ways, with theirconcave sides towards one another. Corresponding reference numerals havebeen used where applicable. In this arrangement, instead of reliancebeing placed on the pressure of the fluid by passing the main seal tobring the emergency seal into action, the emergency seal, or rather eachof them, is brought into action by applying fluid pressure from anexternal source through a connection 24 to the space 12' between the twoseals. Thus in this case the emergency seal is brought into action bytaking a positive step to apply pressure to it rather than simply by thefluid escaping past the main seal.

FIG. 5 shows a further modification of the layout shown in FIG. 4, inthat the pair of emergency seals, again facing opposite ways, isupstream of, i.e. on the pressure side of, the main seal.

Although the preferred embodiments which have been described in detailhas been shown used as an emergency seal backing up a main seal, itwould be possible to use the seal element 8 on its own without a mainseal where the circumstances allow this.

We claim:
 1. A seal assembly between first and second relativelyrotatable elements, said assembly comprising an external cylindricalsurface on said first element, a ring of flexible material which is freeto float axially on said cylindrical surface, said ring having inner andouter annular lips both directed in the same axial direction, aninternal cylindrical surface in said second element disposed around saidring, said lips being capable of engaging and sealing respectivelyagainst the external cylindrical surface on said first element andagainst the internal cylindrical surface in said second element; thedimensions being such that in its free condition, said ring is normallyout of contact with at least one of said surfaces but comes into contactwith both, to form a seal, when fluid pressure acts against that axialface of the ring which faces in the direction towards which said lipsare directed.
 2. The seal assembly set forth in claim 1 wherein saidinner lip is normally clear of said external cylindrical surface butsaid outer lip normally engages said internal surface.
 3. The sealassembly set forth in claim 1 wherein said inner lip normally engagessaid external cylindrical surface but said outer lip is normally clearof said internal surface.
 4. The seal assembly set forth in claim 1wherein said ring includes an intermediate cylindrical portion, morerigid than both lips and extending axially, from its junction with saidlips, in the same axial direction as said lips.
 5. The seal assembly setforth in claim 4 including further a rigid support ring said supportring engaging the ring of flexible material and having an axiallyprojecting portion that engages the cylindrical portion of said ring offlexible material.
 6. The seal assembly set forth in claim 1 whereinthat lip which normally engages its associated surface is, in the ringas manufactured, directed radially, being deflected resiliently fromthat radial direction by its engagement with the said surface.
 7. Theseal assembly set forth in claim 1 wherein said ring is made ofpolytetrafluoroethylene.
 8. The seal assembly set forth in Claim 1,wherein said first element includes a shaft and said second elementincludes a housing in which said shaft is rotatably mounted, incombination therewith, a rotary mechanical face seal carried on saidshaft axially spaced from said seal assembly, the lips on said ringbeing directed towards said rotary mechanical face seal and beingarranged to provide an emergency seal that only comes into action in theevent of failure of said rotary mechanical face seal.
 9. The combinationof seal assemblies set forth in claim 8 including means for detectingand signalling the presence of fluid pressure and/or fluid flow in theregion between said two seal assemblies that indicates failure of therotary mechanical face seal assembly.
 10. The combination of sealassemblies set forth in claim 8 including further a second seal assemblyof the kind set forth in claim 1, facing the opposite way to the firstsuch assembly and arranged between that seal assembly and said rotarymechanical face seal assembly.
 11. The combination of seal assembliesset forth in claim 10 including a connection for allowing fluid pressureto be applied to the region between said first and second sealassemblies.